1. Field of the Invention
The present invention relates to image display devices, and more particularly to an image display device suitable for a personal computer, a word processor, or the like which uses a liquid crystal display as a display unit.
2. Description of the Related Art
Recently personal computers, word processors, and the like have been progressively miniaturized. Accordingly, image display devices employing liquid crystal displays (hereinafter referred to as a LCD) as a display unit, such as in miniaturized and light-weight notebook-sized personal computers, are often used outdoors.
A conventional image display device is described below with reference to FIGS. 6 and 7. FIG. 6 shows a block diagram of the conventional image display device. The image display device includes a manually adjusting operation unit 31, a contrast adjusting unit 32, and a display unit 33.
The manually adjusting operation unit 31, which includes push button switches 31a and 31b, manually adjusts a contrast of the image display device. This operation unit 31 is connected to the contrast adjusting unit 32.
The contrast adjusting unit 32 supplies a control voltage for determining the contrast to the display unit 33. This unit 32 includes a variable resistor and the like (not shown). For example, by operating push button switches 31a and 31b in the manually adjusting operation unit 31, the resistance of the variable resistor in the contrast adjusting unit 32 varies, which as a result causes the contrast of the display unit 33 to vary.
The display unit 33 displays an image signal from, for example, a personal computer (not shown). In addition to the image signal from the personal computer, the control voltage and the like are input from the contrast adjusting unit 32 to the display unit 33.
For example, the relationship between temperature of the display unit 33 and contrast, when using the LCD as the display unit 33, is shown in FIG. 7. When a constant control voltage is supplied to the LCD, the contrast decreases in accordance with an increase in the temperature; when the temperature of the display unit 33 is constant, the contrast decreases in accordance with a decrease in the control voltage.
When the temperature decreases from T.sub.1 to T.sub.2 under the constant control voltage V.sub.1, the contrast increases from C.sub.1 (corresponding to a point P.sub.1) to C.sub.2 (corresponding to a point P.sub.2). To obtain a contrast C.sub.1 at temperature T.sub.2, a control voltage V.sub.2, which is lower than the control voltage V.sub.1, must be supplied to the display unit 33.
Accordingly, by operating the push button switch 31b in the manually adjusting operation unit 31 so as to decrease the control voltage to the control voltage V.sub.2, a standard contrast C.sub.1 (corresponding to a point P.sub.3) is obtained.
On the other hand, when the contrast C.sub.1 (corresponding to the point P.sub.3) is obtained at temperature T.sub.2 under the constant control voltage V.sub.2, the contrast decreases to C.sub.3 (corresponding to a point P.sub.4) in accordance with an increase in the temperature to T.sub.1. In this case, by operating the push button switch 31a in the manually adjusting operation unit 31 so as to increase the control voltage to the control voltage V.sub.1, a standard contrast C.sub.1 (corresponding to the point P.sub.1) is obtained.
As describe above, a change in the temperature under the constant control voltage affects the contrast due to the LCD temperature characteristics. Therefore, whenever the temperature changes, a user is inconvenienced since the user must manually adjust the contrast so that the contrast of the image displayed on the display unit 33 has the most preferable level.